Move scio-core contents to a scala-2 only module

This was done to avoid hacks in build.sbt, and to keep the scio-avro
dependency for Scala 2 scio-test, while removing it for Scala 3.
scio-avro was not yet migrated to SCala 3, as it has some macro
dependencies, that are still waiting for the macro static annotation
support.

Author: jchyb